The trope is based on it allowing for reluctant superhero character. Since powers are given by birth, we can watch kids struggle to come to grips with their powers. If powers are earned, then superheros would all be driven 'winners' ... mostly the psycopaths able to defeat all competitors. That is where we get evil characters.

As a general rule, good guys have powers forced upon them. Bad guys have to work for them. This enforces the concept of an elite class of 'good' families and a dangerous group of upstart peasants. The theme has been with us since ancient times. Far too long imho.